183 research outputs found
Tunneling in a uniform one-dimensional superfluid: emergence of a complex instanton
In a uniform ring-shaped one-dimensional superfluid, quantum fluctuations
that unwind the order parameter need to transfer momentum to quasiparticles
(phonons). We present a detailed calculation of the leading exponential factor
governing the rate of such phonon-assisted tunneling in a weakly-coupled Bose
gas at a low temperature . We also estimate the preexponent. We find that
for small superfluid velocities the -dependence of the rate is given mainly
by , where is the momentum transfer, and is the
phonon speed. At low , this represents a strong suppression of the rate,
compared to the non-uniform case. As a part of our calculation, we identify a
complex instanton, whose analytical continuation to suitable real-time segments
is real and describes formation and decay of coherent quasiparticle states with
nonzero total momenta.Comment: 15 pages, 3 figures; to be published in Phys. Rev.
Statistics and noise in a quantum measurement process
The quantum measurement process by a single-electron transistor or a quantum
point contact coupled to a quantum bit is studied. We find a unified
description of the statistics of the monitored quantity, the current, in the
regime of strong measurement and expect this description to apply for a wide
class of quantum measurements. We derive the probability distributions for the
current and charge in different stages of the process. In the parameter regime
of the strong measurement the current develops a telegraph-noise behavior which
can be detected in the noise spectrum.Comment: 4 pages, 2 figure
Scenario for Ultrarelativistic Nuclear Collisions: Space--Time Picture of Quantum Fluctuations and the Birth of QGP
We study the dynamics of quantum fluctuations which take place at the
earliest stage of high-energy processes and the conditions under which the data
from e-p deep-inelastic scattering may serve as an input for computing the
initial data for heavy-ion collisions at high energies. Our method is
essentially based on the space-time picture of these seemingly different
phenomena. We prove that the ultra-violet renormalization of the virtual loops
does not bring any scale into the problem. The scale appears only in connection
with the collinear cut-off in the evolution equations and is defined by the
physical properties of the final state. In heavy-ion collisions the basic
screening effect is due to the mass of the collective modes (plasmons) in the
dense non-equilibrium quark-gluon system, which is estimated. We avoid the
standard parton phenomenology and suggest a dedicated class of evolution
equations which describe the dynamics of quantum fluctuations in heavy-ion
collisions.Comment: 54 pages, 11 Postscript figures, uses RevTe
Full Frequency Back-Action Spectrum of a Single Electron Transistor during Qubit read-out
We calculate the spectral density of voltage fluctuations in a Single
Electron Transistor (SET), biased to operate in a transport mode where
tunneling events are correlated due to Coulomb interaction. The whole spectrum
from low frequency shot noise to quantum noise at frequencies comparable to the
SET charging energy is considered. We discuss the back-action
during read-out of a charge qubit and conclude that single-shot read-out is
possible using the Radio-Frequency SET.Comment: 4 pages, 5 figures, submitted to PR
Nondemolition measurements of a single quantum spin using Josephson oscillations
We consider a Josephson junction containing a single localized spin 1/2
between conventional singlet superconducting electrodes. We study the spin
dynamics and measurements when a dc-magnetic field acts on
the spin and the junction is embedded into a dissipative circuit. We show that
when tunneling or a voltage are turned on at time the Josephson current
starts to oscillate with an amplitude depending on the initial () value of
the spin -component, . At low temperatures, when effects of
quasiparticles may be neglected, this procedure realizes a
quantum-non-demolition (QND) measurement of .Comment: 4 pages, 1 figure; average value of spin z operator changed to
eigenvalue S_
Spectral Flow, Magnus Force and Mutual Friction via the Geometric Optics Limit of Andreev Reflection
The notion of spectral flow has given new insight into the motion of vortices
in superfluids and superconductors. For a BCS superconductor the spectrum of
low energy vortex core states is largely determined by the geometric optics
limit of Andreev reflection. We use this to follow the evolution of the states
when a stationary vortex is immersed in a transport supercurrent. If the core
spectrum were continuous, spectral flow would convert the momentum flowing into
the core via the Magnus effect into unbound quasiparticles --- thus allowing
the vortex to remain stationary without a pinning potential or other sink for
the inflowing momentum. The discrete nature of the states, however, leads to
Bloch oscillations which thwart the spectral flow. The momentum can escape only
via relaxation processes. Taking these into account permits a physically
transparent derivation of the mutual friction coefficients.Comment: Plain TeX, 19 pages, 5 encapsulated postscript figure
On local invariants of pure three-qubit states
We study invariants of three-qubit states under local unitary
transformations, i.e. functions on the space of entanglement types, which is
known to have dimension 6. We show that there is no set of six independent
polynomial invariants of degree less than or equal to 6, and find such a set
with maximum degree 8. We describe an intrinsic definition of a canonical state
on each orbit, and discuss the (non-polynomial) invariants associated with it.Comment: LateX, 13 pages. Minor typoes corrected. Published versio
Spin and Current Variations in Josephson Junctions
We study the dynamics of a single spin embedded in the tunneling barrier
between two superconductors. As a consequence of pair correlations in the
superconducting state, the spin displays rich and unusual dynamics. To properly
describe the time evolution of the spin we derive the effective Keldysh action
for the spin. The superconducting correlations lead to an effective spin
action, which is non-local in time, leading to unconventional precession. We
further illustrate how the current is modulated by this novel spin dynamics
Quantum dynamics of a model for two Josephson-coupled Bose--Einstein condensates
In this work we investigate the quantum dynamics of a model for two
single-mode Bose--Einstein condensates which are coupled via Josephson
tunneling. Using direct numerical diagonalisation of the Hamiltonian, we
compute the time evolution of the expectation value for the relative particle
number across a wide range of couplings. Our analysis shows that the system
exhibits rich and complex behaviours varying between harmonic and non-harmonic
oscillations, particularly around the threshold coupling between the
delocalised and self-trapping phases. We show that these behaviours are
dependent on both the initial state of the system as well as regime of the
coupling. In addition, a study of the dynamics for the variance of the relative
particle number expectation and the entanglement for different initial states
is presented in detail.Comment: 15 pages, 8 eps figures, accepted in J. Phys.
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